Method of controlling grain structure in alloys and articles produced thereby



Jan. 5, 1937. c. M. CRAIGHEAD 2,067,076

METHOD OF CONTROLLING GRAIN STRUCTURE IN ALLOYS AND ARTICLES PRODUCED THEREBY Filed Feb. 14, 19:4

Z attorney Patented Jan. 5, 1937' UNITED STATES PATET E METHOD OF CONTROLLING GRAN STRUC- TURE IN ALLOYS AND ARTICLES PRO- DUCED THEREBY Charles M. Craighead, New Kensington, Pa, as-

signor to Pittsburgh, Pa.,

Aluminum Company of America,

a corporation of Pennsylvania Application February 14, 1934, Serial No. 711,216

16 Claims.

otherwise deformed and the metal becomes harder. If the metal is now heated to a sufficiently high temperature, recrystallization takes place-that is, the strains are removed and the fragments of the former grains unite to form new grains. Although worked metal consists of broken grains, the size of these fragments is not always uniform or small enough to result in the production of a 'sufilcient number of small grains to secure the desired fine-grained structure in the annealed or heat-treated product. Heretofore the number of crystals or grain fragments, their size andthe condition under which the metal was worked, the heat-treatment and the particular characteristics of the metal being treated, all have had'a bearing upon the grain size of the finished product, andno simple means of accurately controlling the grain size has been known. Consequently, in some cases difliculty has been experienced in the production of certain classes of products due to the excessive grain growth or coarse crystalline structure formed as a result of the fabricating process. Especially has this been true of the fabrication of certain aluminum alloys in the drawn or extruded form. By the term "aluminum used herein and in the appended claims it is meant to include both aluminum and aluminum base alloyscontaining 50 per cent or. more aluminum. The aforementioned difiiculty is particularly likely to occur whenannealed' metal is cold worked a certain critical amount and heat-treated or annealed. This amount 'varies with the alloy being treated, but for most 45 aluminum alloys an'amount of cold work represented by, or equivalent to, a reduction in thickness of a sheet (for example) of 1 to 5 per cent results in the development of a maximum size crystal, occasionally resulting in a roughened surface commonly known in the art as orange peel.

The primary object of this invention is to provide a simple method of controlling grain growth in worked metal surfaces. A further object is to provide a method of positively producing a line crystal structure metals.

I have discovered that during the fabricating process of producing metal articles by mechanical working, thestructural properties of metal, especially grain size, may to a large extent be controlled by an additional step of surface working anda simultaneous production of a plurality of grain fragments or nuclei, before heat-treatment at the recrystallization temperature of the metal.

This invention may form a step in any metal working process wherein the metal is subjected to a mechanical working and to heat-treatment which produces excessive or undesirable grain in heat-treated and worked growth. While the preferred embodiment of my,

invention is identified with controlling grain size in aluminum, this process may be applied to anypther metal or alloy which is subject to mechanical working. For instance, it is applicable to the control of the grain structure of a steel alloy containing up to 20 per centof chromium and up to 12 per cent nickel, commonly known as stainless steel, also to alpha brass, copper, nickel, iron, and zinc.

In the application of this invention to the production of articles by the mechanical working and forming of aluminum followed by thermal treatment of the articles so produced (as by heat vtreating at between 600 and 1000 F.), a secondary working of the surface, such as by abrasion, is

performed intermediate the forming and thermal It is necessary to to a small extent cut the underlying metal. The

preferred form of abrading medium is arevolving steel wire brush which is both very effective and rapid. It has been found that when the abrasion marks or scratches on the surface are close together the grain size will be smaller than when the abrasion marks are farther apart. Hence by regulating the spacing between abrasion, marks the grain size can be effectively controlled. It has also been found that differently shaped grains of a desired maximum size can be produced by different treatments of the surface. That is, if abrasion marks consist of two sets of parallel lines crossing each other at right angles the max,- imum size grain will be substantially square,

other at a 60 angle, the maximum size grain will be triangular. In producing structural articles, the surface is thoroughly abraded to produce a minimum size grain, without regard to the particular shape. It is not necessary to abrade the entire surface of the metal but only those parts in which a control of the grain size is desired.

It has been found that by the method just described not only is the grain of the metal surface controlled, but the grain of the metal below the surface for a. substantial depth is similarly affected. In sheet material of relatively heavy gauge the grain size can be controlled substantially throughout its thickness. This treatment has been effectively applied to sheet of a thickness of 0.064 inch abraded on one surface. If opposite surfaces are abraded, grain control throughoutthe body may be secured .for very substantial thicknesses of metal.

The temperature of heat-treatment which affects grain growth is the temperature at or above which recrystallization of the metal takes place.

However, as is generally known, the higher thetemperature above this critical temperature and the longer the application of heat, the larger are the crystals formed.

It is considered that the abrasion of the surface, assumed for convenience of explanation as consisting of one engraved line on the metal surface, exerts a limiting effect on the grain growth due to portions immediately adjacent the line being slightly disturbed during the act of engraving, producing a slight variation in the grain orientation. It is also considered that the abrading action produces many more and much smaller grain fragments than would ordinarily be present, and the uniformly fine grain structure of this heat-treated product is believed to be largely due to the presence of the increased number of nuclei. The body of the structure whose surface is thus abraded and heat-treated is likewise affected.

In the drawing which forms a part of this specification is illustrated a sheet of aluminum alloy consisting of commercial aluminum containing iron and copper as impurities in an amount of less than 1 per cent and containing 1 per cent of silicon and 0.6 per cent magnesium.

.This sheet was rolled in known manner to a thickness of 0.064 inch and heat-treated to fully relieve the strain. The sheet was then stretched 3 per cent of its length to place the metal in condition to develop unusually large individual crystals upon subsequent heat-treatment. Areas l and 2 were abraded with a revolving scratch brush. Then the sheet was heat-treated for 30 minutes at a temperature of 960 F. The surface was then etched to reveal in a clear manner the remarkable difference in crystal size between the abraded areas I and 2, and the unabraded areas 3 and I. The metal in areas 3 and 4 wasfound to be coarse-grained with an excessively large average size crystal. Areas I and 2, on the other hand, were found to possess a fine-grained metal structure, and a remarkably sharp difference in grain size could be observed on the opposite side of the sheet, which has not been abraded, showing the grain refinement has penetrated the thickness of the sheet.

This invention finds many practical applications. For instance, in the fabrication of a streamline airplane strut of heat-treatable aluminum alloy, a round tube is first drawn and With three sets of parallel lines crossing each completely annealed. Then the tube, without an appreciable change in metal thickness, is brought to ovoid-shaped cross section by drawing through a shaping die. At this point, if the tube is heattreated to develop the strength of the alloy, the metal will develop large crystals due to the strain incurred in the shaping process. Especially is this true of the trailing edge or small end of the ovoid. If the article is further worked, as by stretching slightly to straighten, this condition frequently results in an orange peel surface and the piece must be rejected. This difiiculty is overcome and a tube of uniform fine-grained metal is produced by inserting the step of scratch brushing the shaped surface prior to heat-treating. Another application which finds many and various uses lies in the field of decorating metal surfaces. The method of this invention enables an artist tomake use of his skill in a large degree and to impart to an otherwise plain metal surface a design motif by a predetermined arrangement of the different crystal sizes. In using this method of decorating metal surfaces the metal, after the development of the crystal structure by heat-treating, is etched to make the surface crystal faces visible.

As used in the specification and claims of this patent the term thermal treatment is used to designate any form of anneal or heat treatment which affects the crystalline structure of the metal.

I claim:

1. The method of producing worked and thermally treated metal articles of substantially uniform and controlled, grain size from cast metal, comprising submitting the cast metal to working steps to form a worked and substantially completely shaped metal body, superficially abrading the surface of the worked and shaped body and heating said body up to at least a recrystallizing temperature.

2. In a process of producing worked and thermally treated metal articles, the step of controlling grain growth comprising abrading at least a portion of the surface of a completely shaped worked metal article and then subjecting the article to a thermal treatment at a recrystallizing temperature.

3. As an article of manufacture, a metal object having a decorative grain structure characterized by arrangement of large and small crystalline grains in accordance with a'design motif.

4. As an article of manufacture, a metal object having a decorative grain structure characterized by metal crystals in the plane of the decorative surface having regular and controlled rain boundaries.

5. As an article of manufacture, a metal object having a decorative grain structure characterized by metal crystals having in the plane of the surface of the object a maximum crystal size with a regular controlled boundary. 7

6. The process of producing a worked and thermally treated aluminum article having grain growth controlled at least inportions adjacent a surface thereof, comprising abrading said surface and then heating the article at least to a re-crystallizing temperature.

7. The process of producing a worked and thermally treated aluminum article having grain growth controlled over portions of its surface, comprising abrading said portions of the surface of a worked aluminum article and then subjecting the article to thermal treatment at a recrystallizing temperature.

8. In a process of producing a worked and thermally treated aluminum article, the step of controlling grain growth comprisingsuperficially abrading at least a portion of the surface of the substantially completely worked article, and thensubjecting it to thermal treatment.

9. The process of producing a worked and thermally treated metal article having controlled grain growth over portions of its surface, comprising working metal into substantially the desired final shape, then superficially abrading said portions of its surface, and then subjecting the article to thermal treatment at a re-crystallizing temperature.

10. The process of producing a Worked, thermally treated metal article having at least partially controlled grain size, comprising casting the metal, working the cast metal and shaping it to substantially the desired final form, superficially abrading the portions of the surface of the shaped article where controlled grain size is desired, and then heating the article at least to a re-crystallizing temperature.

11. The process of producing a worked and thermally treated metal article comprising an-- nealing a substantially. completely worked metal article, abrading at least portions ofits surface, and subjecting the article to heat treatment at a re-crystallizing temperature.

12. The process of producing a worked and thermally treated metal article comprising annealing a substantially completely worked metal article, subjecting it tor'elatively small amountv of additional working, abrading at least portions of its surface, and subjecting the article to heat treatment at a re-crystallizing temperature.

13. The process of producinga worked and thermally treated metal article of controlled grain size comprising working metal into an '3 article of substantially completed form, annealing the article, subjecting it to additional working to an extent normally conducive to excessive grain growth, abrading at least portions of the ,surface of the article, and heating the article at least to a re-crystallizing temperature of the metal.

14. The process of producing a decorative worked and thermally treated aluminum article, comprising working aluminum into an article of substantially completed form, annealing the article, subjecting it to additional Working to an extent normally conducive to excessive grain growth during a subsequent thermal treatment, abrading portions of the surface of the article, heating the article at least to a re-crystallizing temperature, and etching the surface of the article to bring out the resulting contrasting grain structure.

15. In a process of producing a worked and thermally treated metal article having a composition subject to excessive grain growth under the conditions of production, the step of controlling conditions as are normally conducive to excessive grain growth, which comprises superficially rcughening the surface of the article to produce thereon a plurality of grain fragments or nuclei, and subjecting the article to heat treatment at a recrystallizing temperature. 1

- CHARLES M. CRAIGHEAD. 

